The present invention relates to optical encoders and, more particularly, to encoders which may be used to provide print control information in an ink jet copier for copying all types of materials, including printed text and photographs. In recent years ink jet printing has developed in sophistication to the point where it is now possible to print both text material and other types of printed material, such as photographs with a high degree of resolution. A technique similar to half-tone printing is used to reproduce photographs and other graphics. The photograph is reproduced by depositing ink drops closely together in areas which are to be dark in tone and further apart in areas which are to be lighter in tone. This does not correspond precisely to half-tone printing, since in that technique the dots are uniformly spaced between centers but are of differing size. The resulting image is quite similar in effect, however.
In order to utilize an ink jet printer as the printing mechanism for a copier, it is necessary to have some method of scanning the master to produce print control information which will space the ink drops the appropriate distance apart in order to reproduce the master. It is known in the prior art to illuminate a master and then, by means of a rotating mirror or other optical arrangement, direct the reflected light from along a scan line on the master to one or more optical lenses which, in turn, direct this light to a photoelectric transducer. The output signal from the photoelectric transducer provides an indication of the printed material along the scan line of the master, with more light being reflected from the lighter toned areas, indicating little or no ink deposits in these areas. The master may be scanned a number of times along parallel closely spaced scan lines until sufficient information is obtained to control the print operation. Alternatively, a stationary optical configuration may be used, as shown in U.S. Pat. No. 3,604,846, issued Sept. 14, 1971, to Behane et al, and assigned to the assignee of the present invention. In the Behane et al encoder, light is transmitted through the master, with the master being moved with respect to the optical system.
Another scanning arrangement is shown in U.S. Pat. No. 3,928,718, issued Dec. 23, 1975, to Sagae et al. In the Sagae et al device, the master is placed on a rotating drum. The optical scanning arrangement is slowly moved axially along the rapidly rotating drum, thus scanning a plurality of scan lines on the master. Various techniques have been used to convert the analog output from the photoelectric transducer associated with the scanner into a form useful in controlling print operations. One technique disclosed in U.S. Pat. No. 3,580,995, issued May 25, 1971, to Klensch, is to use the analog output to control a variable oscillator. The sheet upon which the copy is to be printed in scanned by the printing mechanism (here a cathode ray tube) in synchronism with the scanning of the master and, upon each successive output pulse from the oscillator, a dot is printed on the copy sheet. After the entire image has been scanned along a number of parallel scan lines on the master and on the copy, the copy will resemble a half-tone reproduction of the master.
As suggested in U.S. Pat. No. 3,373,437, issued Mar. 12, 1968, to Sweet et al, an ink jet printer lends itself quite well to printing images in a range of tones, such images being supplied in the form of video or facsimile signals. The drops supplied to the copy medium by the printer are of relatively small size and their deposit may be precisely controlled. In the Sweet et al device, the drops are counted out in correspondence to the gray level desired. The circuit operates on a pulse stretching technique in which the pulse width is proportional to the gray level.
A problem with scanning and encoding image information accurately occurs when a master is scanned which includes thin lines perpendicular to the scan line. If any integration is involved in encoding information, such as a divide-by-N counter on a variable oscillator output, it is quite possible that these lines will not be printed or that they will be distorted drastically.
U.S. Pat. No. 1,790,723, issued Feb. 3, 1931, to Ranger, discloses facsimile encoding and transmission systems in which the copy is scanned along a series of parallel, closely spaced, scan lines. Print information is encoded into a series of pulses which may be transmitted over a telegraph or telephone line. Each pulse will result in a drop or a line of ink being placed on the copy along the copy scan line. The facsimile picture is built up from a series of such dots and lines. The encoding technique used by Ranger is one in which a pair of capacitors are alternately charged from an analog signal supplied by a photoelectric transducer. The capacitors act as integrators and cause an output signal to be switched between two potentials each time a sufficient voltage is impressed upon one of the capacitors. Dots and lines are placed on the copy under control of this pulse train output signal. In an improvement patent to Ranger, U.S. Pat. No. 1,973,726, issued Sept. 18, 1934, an attenuation to the integrating capacitors is provided in the circuit during normal operations. When, however, a sudden transition occurs, this attenuation is bypassed and printing will occur much more rapidly. While improving the definition of the resulting facsimile picture, the bypass capacitors used in the improved Ranger device cause a significant increase in the noise in the system, since they are acting as differentiators.
Thus, it is seen that there is a need for a method and apparatus for improving the encoding of print information for an ink jet copier.